1. Field of the Invention
The present invention relates to a light controlling sheet and a surface light source device for use in illumination of a liquid crystal device.
2. Background Art
As the light controlling sheet, a sheet on which rectangular prisms are arranged in larger numbers is known.
In such a sheet on which rectangular prisms are arranged, however, light tends to be radiated in an unduly wider range of the outgoing angular direction.
As used herein, the terms “incident light” and “outgoing light” are intended to mean angles relative to a sheet surface (a surface when viewed as the entire surface of a light controlling sheet), and express angles each defined by the normal line of the sheet surface and the direction along which light travels, respectively.
To prevent light from being radiated in such an unduly wider range of the outgoing angular direction, examples in which a lens is provided on the outgoing light side while a shielding portion corresponding to the lens on the outgoing light side is arranged on the incident light side are described in Patent Documents 1 to 4.
However, even in the cases of the lens sheets described in Patent Documents 1 to 4, unless the lens and the shielding portion are formed in an appropriate form, a great amount of light may be generated that can be radiated in an unduly wide range of the outgoing angular direction, thus deteriorating the utilization efficiency of light.
Any of the lens sheets described in Patent Documents 1 to 4 has a flat surface on its incident light side. Therefore, when it is superimposed on a separate optical sheet having a flat surface, Newton rings may tend to occur.
Attempting to make the utilization efficiency of light higher, only the utilization of the lens on the outgoing light side and the shielding portion on the incident light side may significantly restrict the effect. Accordingly, such systems can not satisfy the need to achieve a satisfactorily higher level of light utilization.
Furthermore, the shielding portion provided on the incident light side requires critically accurate alignment relative to the position of the lens on the outgoing light side, thus making its formation significantly difficult.
Patent Document Cited:
    Patent Document 1: TOKUKAIHEI No. 8-95038, KOHO    Patent Document 2: TOKUKAIHEI No. 10-241434, KOHO    Patent Document 3: TOKUKAI No. 2000-284268, KOHO    Patent Document 4: TOKUKAI No. 2005-221619, KOHO